The $99 Apple Developer account allows you to test your iOS apps on a maximum of
100 devices of each type:

AppleTV100AppleWatch100iPad100iPhone100iPodTouch100

And you must register the UDID of each device in your Apple developer account.
If you have the $299 Enterprise account then you can install your app on any
device without registering it.

The easiest way to find UDIDs is to connect to your iTunes account. Then connect
your iOS device to your Mac computer. Your device will appear on the left
sidebar in iTunes. Click on this to display your device information. Then click
on the serial number, and you will see your UDID.

Return to your account on Developer.apple.com, go to IOS Apps >
Devices > All, and click the plus button on the top right to register a new
device. You can make the name anything you want, and the UDID must be the UDID
copied from iTunes.

If you have a large number of devices to register, you may enter them in a text
file in this format, and then upload the file: